This invention relates to an electronic impedance matching device. More particularly, the present invention relates to microstrip coupled transmission lines used to form electronic impedance transformers on low temperature co-fired ceramics.
Impedance matching circuits are used to efficiently transfer energy between electronic circuits, which are electrically connected to each other and have different characteristic impedances. Impedance matching is accomplished by rendering the impedance seen at the output of one circuit equal to the impedance seen at the input of another interconnected circuit. To this end, it is necessary to match source impedances and load impedances of the circuits. The matching circuit is located between the output of the original source circuit and the input of the original load impedance. The matching circuit acts to transform the original unmatched interface into two matched interfaces. When the impedances are matched, maximum power can be provided from the power source to the load.
The most common form of broadband impedance matching network employs wire wound inductors sharing a common magnetic path that incorporates special ferromagnetic magnetic materials to greatly increase the mutual magnetic coupling between the inductors. The physical magnetic material path is commonly called a magnetic core. The best of these magnetic materials can increase the magnetic coupling by more than 10,000:1 relative to air or other non-ferromagnetic materials. These impedance matching devices, commonly called transformers, provide ratio-metrically broadband results at low power line and audio frequencies. The broadband performance depends on the magnetic coupling enhancement provided by the special high permeability magnetic materials. At high radio frequencies (hereinafter referred to as xe2x80x9cRFxe2x80x9d) these materials lose most of their high magnetic permeability properties so this simple approach to broadband impedance matching becomes ineffective. Further, these devices are limited at high RF frequencies to applications wherein the minimum impedance is approximately 10 xcexa9. For these lower impedance applications the small round transformer wires add excessive undesired leakage inductance.
For {RF} impedance matching below 10 xcexa9, another type of impedance matching device is necessary. Impedance matching is necessary, for example, for wireless communication systems where it is desired to impedance match a low impedance power amplifier to a high impedance antenna, wherein the impedance of the power amplifier can be 3.125 xcexa9 and the impedance of the antenna is typically 50 xcexa9 (i.e. 16:1 transformer). The load impedance of a power amplifier in a transmitter output stage is adjusted to match with the input impedance of an antenna to achieve maximum efficiency of the transmitter output stage (ratio of the power fed to the antenna to the overall power used). The applications call for power amplifiers with high output power operating at low DC power supply voltages. This leads to power amplifiers that operate into a load of a few ohms. If the impedance of the power amplifier is not matched with the impedance of the load device while supplying a high-frequency power, then the RF power efficiency is low.
With the progress of electronic and communication technologies, the consumers of electronic communication products demand higher quality services and, in particular, desire to be provided with various services by a single product. To accede to this demand, various electronic circuits having different characteristics have come to be provided in a single communication product. Accordingly, there is a demand for an impedance matching circuit capable of matching impedances at multiple frequencies. Conventional power amplifiers provide the impedance matching capability in a narrowband way that is incapable of simultaneously operating at the desired multiple frequencies.
Without some form of broadband impedance matching network, multi-band radios would require a multiplicity of narrow, single band RF power amplifiers, which utilize narrow band impedance matching circuits. Because multi-band radios use multiple RF power amplifiers, they are typically large in size and expensive to fabricate. In addition to performing the impedance matching function, the power amplifier output network commonly must also perform a direct current (hereinafter referred to as xe2x80x9cDCxe2x80x9d) bias network function. The DC bias network function requires that DC power from an external DC power supply be connected to the amplifier""s output transistor while simultaneously connecting the RF output of the transistor to and through the impedance matching network so that the RF but not the DC reaches the external RF load. The DC bias function of a single band power amplifier typically requires a large shunt inductor to stop the RF signal while passing the DC bias current to a large bypass capacitor to ground whose low RF impedance reduces the remaining RF voltage leaving primarily DC voltage to carry the DC supply current to the amplifier""s DC power supply. Further, a large series capacitor is typically used to stop the power amplifiers DC power supply voltage output from reaching the RF output signal load of the amplifier. The undesired reactive parasites associated with the large shunt inductor and the large series capacitor can further reduce the bandwidth of the associated matching network.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
To achieve the objects and advantages of the invention, a new and improved broadband impedance matching integrated circuit is disclosed. In the preferred and initial embodiment, the impedance matching integrated circuit is formed within a low temperature co-fired ceramic (hereinafter referred to as xe2x80x9cLTCCxe2x80x9d). The integrated circuit includes an electrically conductive ground plane with multiple electrically non-conductive dielectric layers. The generalized LTCC circuit function is created with selectively patterned conductors between the dielectric layers. Required between layer conductivity is achieved with vias each consisting of a small conductor filled hole through the dielectric layer. In an electrical circuit the electrical elements or components are directly or indirectly connected to an electrical ground, which by definition is at zero volts. In the initial embodiment, which employs microstrip format construction, the electrical ground becomes a ground plane conductor that covers the bottom surface of the LTCC substrate. In the microstrip format the top surface of the LTCC substrate is ideally open to unlimited space of air or other dielectric. If the stripline format had been used the top surface of the LTCC substrate would also have been covered with a conductor to create a second ground plane. The inventive microstrip circuit embodiment contains a dielectric layer positioned on the ground plane, a DC bias plane positioned on the dielectric layer, and a high permittivity dielectric material layer positioned on the DC bias plane. An auxiliary ground plane is positioned on the high permittivity dielectric material layer, wherein the auxiliary ground plane is electrically isolated to a direct current from the DC bias plane. The auxiliary ground plane is electrically connected to the ground plane through electrically conductive vias. Further, it will be understood that while the DC and auxiliary ground planes are electrically isolated and separate, they are also approximately at the same RF voltage potential. It should be noted that in this specific embodiment the close proximity of the auxiliary ground plane and the ground plane to the DC bias plane combined with the high permittivity dielectric material layer between them creates a large electric capacity between them that shorts the RF voltage on the DC bias plane to the ground plane. Because of this capacity the DC bias plane has a DC bias voltage but little RF voltage and the DC bias plane approximates an RF ground.
A first conductive transmission line with a width is positioned a distance from the auxiliary ground plane and the DC bias plane. A dielectric material layer with a thickness is positioned on the first conductive transmission line. A second conductive transmission line with a width is positioned on the dielectric material layer. The first and second conductive transmission lines are in close enough proximity so that their electromagnetic fields have significant interaction. The first and second conductive transmission lines are interconnected at the ends so that a current is capable of flowing anti-parallel through the first and second conductive transmission lines and wherein the first and second conductive transmission lines behave as an electro magnetically coupled tapped autotransformer. The via, connecting the first and second transmission lines together, becomes the tap of the autotransformer.
Also, the first conductive transmission line is interconnected at one end with conductive vias to the DC bias plane to provide an RF ground path while passing an input DC bias current and to separate the DC bias current from an RF signal. Further, in the preferred embodiment, the DC bias plane is interconnected with the first conductive transmission line with a plurality of conductive vias, which reduce an undesired inductance to the DC bias plane, which also acts as an RF ground.